To "solve" an autostereogram (Magic Eye) clue the viewer must shift their focus to bring out the 3D image "hidden" in the pattern. The example below shows a shark:
A really easy way to create one is to use an online tool such as https://www.easystereogrambuilder.com which allows you to type your clue in its own generator and then download the image. You can can then show the image to the treasure hunters on a tablet or print it out.
For more information about using autostereograms as a treasure hunt clue see our Magic Eye post.